The present invention relates to the field of mass spectrometry and in particular ion mobility spectrometry.
Ion Mobility Spectrometry (“IMS”) is a well established analytical technique where ionic species are separated according to their ion mobility by subjecting the ions to a weak electric field in the presence of a buffer gas. A known ion mobility spectrometer comprises a linear tube filled with gas. A static homogeneous axial electric field is maintained along the length of the tube. Ions experience an axial force in one direction due to the electric field and an effective force in the other direction due to collisions with the buffer gas.
The ion mobility resolution R of such a system can be expressed by Eqn. 1:
                    R        =                              T                          Δ              ⁢                                                          ⁢              t                                =                                    K                              Δ                ⁢                                                                  ⁢                K                                      =                                          LEq                                  16                  ⁢                                                                          ⁢                  kT                  ⁢                                                                          ⁢                                      ln                    ⁡                                          (                      2                      )                                                                                                                              (        1        )            wherein L is the length of the tube (m), E is the electric field (V/m), K is Boltzmann's constant, T is the temperature of the buffer gas (K), K is the mobility (M2V−1s−1) and q is the charge on the ion.
To improve ion mobility resolution R the length of the drift tube may be increased or the electric field may be increased. However, the relationship in Eqn. 1 only holds approximately below a low electric field limit wherein the ratio of electric field to buffer gas number density is below a certain value. To allow the field to be increased without exceeding this value requires the pressure to be increased by the same factor. Both these approaches lead to practical limitations in the IMS resolution which can ultimately be achieved.
Another approach to increasing IMS resolution without increasing path length is described in Novel Ion Mobility Setup Combined with Collision Cell and Time of Flight Mass Spectrometer, J. Am Soc Mass Spectrom, 2006, Volume 17, Issue 5, p 691-699, Alexander Loboda. In this method the buffer gas is allowed to flow in a direction opposing the electric field. The combination of gas flow and DC field allow ions to remain longer in the cell thereby experiencing more collisions with the buffer gas. This results in marked improvements in mobility resolution without increasing the physical length of the mobility device. Careful design of the gas flow dynamics of the IMS cell must be considered in this approach to avoid turbulent flow effects which will cause a degrading of IMS resolution.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.